Magnetic recording medium

ABSTRACT

A magnetic recording medium characterized by a magnetic layer comprising (A) magnetic fine particles of plate-like hexagonal ferrite having a particle size of 0.3 microns or less and an easily magnetizable axis in the direction vertical to the flat plate surface (B) a polyurethane resin having a metal sulfonate group and (C) one or more esters of C 10  -C 18  fatty acids with alcohols having not more than 15 carbon atoms.

The present invention relates to a magnetic recording medium producedwith hexagonal ferrite as a magnetic material in which the surface ofcoating film (magnetic layer) is superior in smoothness, particularlythe friction coefficient of the surface is low and changes little over awide range of temperatures and humidities, and besides the abrasionresistance and durability are excellent. Particularly, the presentinvention relates to a flexible and disc-like magnetic recording medium.

Hitherto, recording and reproduction on magnetic recording media aregenerally carried out by magnetization in a direction parallel to thesurface of the media. For specific examples of the magnetic body, thereare given γ-Fe₂ O₃, CrO₂, cobalt ferrite (CrO₂.Fe₂ O₃), iron oxidecontaining adsorbed cobalt, etc. With magnetic recording mediacontaining these magnetic bodies, when high-density recording is triedby increasing the recording frequency to shorten the wave length, thereappears a reduction in the reproduced output by a demagnetizationaction, the magnetic recording media becoming fundamentally unsuitablefor use in high-density recording. On the other hand, when magnetichexagonal ferrite powders, particularly, in the form of a flat platehaving an easily magnetizable axis in the direction vertical to itssurface are used, magnetization in the direction vertical to the surfaceof magnetic recording media becomes possible, so that thedemagnetization action is difficult to occur, and besides high-densityrecording is possible by the use of common ring heads.

In the prior-art methods (longitudinal magnetization), it was difficultto obtain magnetic recording media superior in both dispersibility ofmagnetic fine powders and durability. Similarly, dispersion of fine andflat plate-like substances like the plate-like hexagonal ferrite used inthe novel method (vertical magnetization) is more difficult, and whenthe dispersibility is improved, there appear a reduction in the strengthof coating film as well as blooming and adhesiveness at elevatedtemperatures. Consequently, magnetic recording media having smoothsurface and good durability are not yet obtained.

When flat plate-like fine magnetic powders are used, uniform dispersionof the powders by the use of the generally known polyurethane resins,epoxy resins, nitrocellulose, vinyl chloride/vinyl acetate copolymers,polyester resins, etc. is very difficult. This not only causes a veryrough surface, a great loss in the reproduction of short wave lengths ona ring head and a great hindrance to high-density recording, but alsolowers abrasion resistance and durability. Consequently, a method ofadding dispersing agents such as surface active agents, etc. has beenused. With this method, however, dispersion of plate-like hexagonalferrite fine particles of 0.3 μm or less in particle size is not onlyinsufficient, but addition of surface active agents weakens adhesion ofthe magnetic layer to the non-magnetic substrate and as a result, causesthe coating film (magnetic layer) to peel off upon long-term driving.

Previously, we invented a binder superior in the dispersion of magneticpowders as well as adhesion force to non-magnetic substrates (JapanesePatent Application No. 66459/1978). When, however, the plate-likehexagonal ferrite fine particles of the present invention having aparticle size of 0.3 μm or less are used, the invention above gave onlymagnetic recording media which are poor in abrasion resistance anddurability, because dispersion of the fine powders in the binder isinsufficient.

The present inventors extensively studied to obtain magnetic recordingmedia having a magnetic layer which is suitable for high-densityrecording, has a smooth surface and a friction coefficient which is lowand changes little over a wide range of temperatures and humidities, andbesides which is superior in abrasion resistance and durability. As aresult, the present inventors solved the foregoing problems andsucceeded in obtaining magnetic recording media using fine magneticpowders of plate-like hexagonal ferrite which are suitable forhigh-density recording and have excellent surface smoothness and afriction coefficient which is low and changes little over a wide rangeof temperatures and humidities as well as excellent abrasion resistanceand durability.

The present invention provides a magnetic recording medium characterizedin that a magnetic layer containing (A) magnetic fine particles ofplate-like hexagonal ferrite having a particle size of 0.3 μm or lessand an easily magnetizable axis in the direction vertical to the flatplate surface, (B) a polyurethane resin having a metal sulfonate groupof 20 to 500 equivalents/10⁶ g of the polymer and (C) one or moremembers selected from the esters of C₁₀ -C₁₈ fatty acids with alcoholshaving not more than 15 carbon atoms, the ratio of (C) to (A) being 0.1to 20 wt. %, is applied to a non-magnetic substrate.

Plate-like hexagonal ferrite used in the present invention isrepresented by the general formula, AFe_(12-x) M_(x) O₁₉ in which Arepresents Ba, Sr, Pb or Ca, M represents Co, Ti, Ni, Mn or Nb, and xrepresents 0 to 2.5, and it has an easily magnetizable axis in thedirection vertical to the flat plate surface and a particle size of 0.3μm or less. Hexagonal ferrite having a particle size of larger than 0.3μm is not only poor in medium noise but also unsuitable for high-densityrecording.

It suffices for the polyurethane resin used in the present invention tocontain a metal sulfonate group of 20 to 500 equivalents/ 10⁶ g of thepolymer. When the content of the metal sulfonate group is less than 20equivalents/10⁶ g of the polymer, no increase in the Br/Bm ratio can notonly be expected, but high-density packing of magnetic particles cannotalso be attained. When the content of the metal sulfonate group exceeds500 equivalents/10⁶ g of the polymer, the polyurethane resin becomespoor in solubility in solvents, lacking a practical value.

The polyurethane resin of the present invention is obtained by thereaction of a polyhydroxy compound with polyisocyanate, in which case apart or all of the polyhydroxy compound used contain a metal sulfonategroup.

A particularly preferred example of the polyhydroxy compound having ametal sulfonate group is a polyester polyol having a metal sulfonategroup which comprises a carboxylic acid component having no metalsulfonate group, a glycol component and a dicarboxylic acid componenthaving a metal sulfonate group.

The carboxylic acid component having no metal sulfonate group includesaromatic dicarboxylic acids (e.g. terephthalic acid, isophthalic acid,orthophthalic acid, 1,5-naphthalic acid), aromatic oxycarboxylic acids[e.g. p-oxybenzoic acid, p-(hydroxyethoxy)benzoic acid], aliphaticdicarboxylic acids (e.g. succinic acid, adipic acid, azelaic acid,sebacic acid, dodecanedicarboxylic acid), and tri- or tetracarboxylicacids (e.g. trimellitic acid, trimesic acid, pyromellitic acid).

The glycol component includes ethylene glycol, propylene glycol,1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,neopentyl glycol, diethylene glycol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, ethyleneoxide and propylene oxide adducts of bisphenol A, ethylene oxide andpropylene oxide adducts of hydrogenated bisphenol A, polyethyleneglycol, polypropylene glycol, polytetramethylene glycol, etc. Thisglycol component may be used in combination with a tri- or tetraol (e.g.trimethylolethane, trimethylolpropane, glycerin, pentaerythritol).

The dicarboxylic acid component having a metal sulfonate group includes5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid,2-sodium sulfoterephthalic acid, 2-potassium sulfoterephthalic acid,etc. The copolymerization amount of these dicarboxylic acid componentshaving a metal sulfonate group is 0.5 mole % or more, preferably 1 to 50mole % based on the total carboxylic acid components.

The foregoing polyhydroxy compounds having a metal sulfonate group maybe used alone or in combination. These polyhydroxy compounds may be usedtogether with a polyhydroxy compound having no metal sulfonate group,such as the common polyester polyols, polyether polyols or acrylpolyols, derivatives of castor oil or tall oil and other hydroxygroup-containing compounds.

The polyisocyanate used for the preparation of the polyurethane resin ofthe present invention includes 2,4-tolylenediisocyanate,2,6-tolylenediisocyanate, p-phenylenediisocyanate,diphenylmethanediisocyanate, m-phenylenediisocyanate,hexamethylenediisocyanate, tetramethylenediisocyanate,3,3'-dimethoxy-4,4'-biphenylenediisocyanate,2,4-naphthalenediisocyanate, 3,3'-dimethyl-4,4'-biphenylenediisocyanate,4,4'-diphenylenediisocyanate, 4,4'-diisocyanatediphenyl ether,1,5-naphthalenediisocyanate, p-xylylenediisocyanate,m-xylylenediisocyanate, 1,3-diisocyanatemethylcyclohexane,1,4-diisocyanatemethylcyclohexane, 4,4'-diisocyanatedicyclohexane,4,4'-diisocyanatedicyclohexylmethane, isophoronediisocyanate, etc. Ifnecessary, these polyisocyanates may be used together with small amountsof 2,4,4'-triisocyanatediphenyl, benzenetriisocyanate, etc.

The polyurethane resin is obtained by reacting a polyhydroxy compoundwith polyisocyanate with or without a solvent by the well-known method.Preferred mixing ratio of polyisocyanate to polyhydroxy compound is0.5-2 to 1 as expressed by the ratio of the NCO group of polyisocyanateto the OH group of polyhydroxy compound. The molecular weight of thepolyurethane resin obtained is preferably 8,000 to 100,000.

So far as the dispersibility of the plate-like fine particles ofhexagonal ferrite and the object of the present invention are notdamaged, to the polyurethane resin of the present invention may be addeda resin compatible with it and/or a compound crosslinkable with it. Theamount of such resin or compound added is generally 2 to 100 parts byweight based on 100 parts by weight of the polyurethane resin.

The resin compatible with the polyurethane resin includes polyvinylchloride resins, polyester resins, cellulosic resins, etc. The compoundcrosslinkable with the polyurethane resin includes epoxy resins,isocyanate compounds, melamine resins, urea resins, polyol compounds,etc. Of these compounds, isocyanate compounds are particularlypreferred.

Specific examples of the esters of C₁₀ -C₁₈ fatty acids with alcoholshaving not more than 15 carbon atoms which act as a lubricant in thepresent invention include n-butyl stearate, octyl stearate, tridecylstearate, n-butyl palmitate, n-butyl myristate, n-butyl laurate, n-decylmyristate, myristyl palmitate, ethyl n-nonadecylate, n-propyl laurate,etc. Of these esters, preferred ones are those obtained withstraight-chain alcohols having 4 to 15 (both inclusive) carbon atoms,more preferred ones are those obtained with straight-chain saturatedalcohols having 4 to 15 (both inclusive) carbon atoms, and particularlypreferred ones are butylcellosolve stearate and butylcellosolvepalmitate. These lubricants may be used alone or in mixture of two ormore of them. The amount of the lubricant blended is preferably 0.1 to20 wt. %, more preferably 0.1 to 10 wt. %, particularly preferably 1 to7 wt. % based on the magnetic powders. When the amount is less than 0.1wt. %, the intended effect is not obtained, while when the amount islarger than 20 wt. %, it sometimes occurs that blooming is generated atelevated temperatures to stain magnetic heads.

The details of a reason why the magnetic recording medium of the presentinvention is obtained when the foregoing particular lubricant is used incombination with the polyurethane resin having a metal sulfonate groupof 20 to 500 equivalents/10⁶ g of the polymer, are not clear. But thefollowing may be considered: Because of good affinity and compatibilitybetween the lubricant and polyurethane resin of the present invention,dispersion of the ferromagnetic fine particles of flat plate-likehexagonal ferrite becomes good and separation of the lubricant isinhibited by combining the lubricant and polyurethane resin, which isuseful to obtain magnetic recording media for high-density recording.For incorporating the lubricant in the magnetic layer, there are methodssuch as a method of kneading the magnetic particles and the bindertogether, a method of dissolving the lubricant in a solvent, coating theresulting solution onto the magnetic layer and drying, a method ofdipping the magnetic layer in said solution and drying, and combinationsof these methods. These methods are properly selected as need arises.

Non-magnetic powders used in the present invention having a Mohs'hardness of 6 or more and a particle size of 1 μm or less include TiO₂,SiO₂, Al₂ O₃, Cr₂ O₃ and α-Fe₂ O₃. When the particle size is larger than1 μm, the surface property becomes poor to make medium noise large, sothat the particle size needs to be 1 μm or less. When the Mohs' hardnessis less than 6, there is little effect to improve the abrasionresistance and durability of the surface.

These non-magnetic powders having a Mohs' hardness of 6 or more may beadded alone or in mixture of two or more of them. The amount of thepowders added is 2 to 20 wt. %, preferably 2 to 10 wt. % based on themagnetic powders. When the amount is less than 2 wt. %, the frictioncoefficient of the coating film is not sufficiently low, andsatisfactory durability is not obtained. While when the amount is morethan 20 wt. %, the durability surely improves, but, because the amountbecomes too large relative to that of the magnetic powders, nosufficient output is not only obtained, but abrasion of the headsometimes occurs to cause a reduction in reproduced output. As describedabove, the present inventors found that magnetic recording media havingexcellent surface smoothness, a friction coefficient which is low andchanges little over a wide range of temperatures and humidities andexcellent durability can be obtained only when plate-like hexagonalferrite particles having a particle size of 0.3 μm or less suitable forhigh-density recording are used in combination with the particularbinder, particular lubricant and non-magnetic powders having a properhardness. The non-magnetic substrate used in the present inventionincludes films, sheets, tapes, etc. comprising polymers such aspolyethylene terephthalate, polyphenylene sulfide, polypropylene,cellulose diacetate, polyimide, polyamide, polycarbonate, polyvinylchloride, etc. To these polymers may be added reinforcing agents,fillers, lubricants, etc.

Further, in order to raise the electroconductivity of the presentmagnetic recording medium, fine powders of carbon and/or graphite may beadded. Also, in order to raise the stability of the present binder,antioxidants, etc. may be added.

The present invention will be illustrated with reference to thefollowing examples, but it is not limited to these examples.

EXAMPLES 1 TO 7 AND COMPARATIVE EXAMPLES 1 TO 3

    ______________________________________                                                                   Part by                                                                       weight                                             ______________________________________                                            Plate-like hexagonal ferrite (particle size, 0.2 μm)                                                    100                                              Polyurethane resin (described in Table 1)                                                                  20                                               Nitrocellulose (a product of Daiseru Kagaku                                                                5                                                Kogyo Co.)                                                                    Al.sub.2 O.sub.3 (Mohs' hardness, 9; particle size, 0.1                                                    6mu.m)                                           Carbon black (a product of Lion Akzo Co., Ltd.)                                                            4                                                Coronate L (a product of Nippon Polyurethane Co.)                                                          5                                                Methyl ethyl ketone          200                                              Toluene                      200                                              n-Butyl stearate             3                                            ______________________________________                                    

These materials were thoroughly milled together by means of a ball millto prepare a magnetic paint. This paint was coated onto the bothsurfaces of a polyethylene terephthalate film of 75 μm in thickness sothat the dry thickness was 1.5 μm, dried and hardened by allowing thefilm to stand at a temperature of 60° C. for 24 hours. Magnetic discswere then stamped out of the film and evaluated. The results are shownin Table 1.

The magnetic discs were evaluated by the following methods in theexamples and comparative examples described later:

Surface roughness:

Measured on "Surfcom 300A" (a surface roughness meter, a product ofTokyo Seimitsu K.K.).

Durability:

A continuous driving test was carried out by magnetic disc drive in anatmosphere controlling box, and output reduction (%) after the3,000,000th passage was measured.

*: Magnetic layer peels off or driving stops by over torque.

**: Not measurable.

Void content (%):

Expressed by the vol. % of voids in the magnetic layer. VoidContent=1-Ms/q.Mo

Ms: saturation magnetization of magnetic layer in unit volume.

q: Vol. % of magnetic powder to total volume of components composingmagnetic layer.

Mo: saturation magnetization of magnetic powder in unit volume.

Dynamic friction coefficient:

Friction generated when a head load of 20 g was applied to a floppy discwhich was being rotated at 300 rpm was detected and measured by means ofa strain gauge.

                                      TABLE 1                                     __________________________________________________________________________                     Metal sulfonate group                                                                     Surface                                                   Kind of poly-                                                                         content of polyurethane                                                                   roughness                                                                           Durability                                 No.      urethane resin                                                                        resin (equivalents/10.sup.6 g)                                                            Ra (μm)                                                                          5° C.                                                                     45° C.                           __________________________________________________________________________    Example No.                                                                          1 a        26         0.025 5 >                                                                              7                                              2 b        47         0.019 5> 8                                              3 c       150         0.018 5> 7                                              4 d       370         0.018 5> 6                                              5 e       500         0.020 7  12                                             6 a/c = 2/1                                                                             --          0.018 5> 8                                              7 b/c = 1.2                                                                             --          0.019 5> 8                                       Comparative                                                                          1 n        0          0.039 *  *                                       example No.                                                                          2 m        5          0.036 *  *                                              3 p       1100        **    ** **                                      __________________________________________________________________________

EXAMPLES 8 TO 15 AND COMPARATIVE EXAMPLES 4 TO 7

Magnetic discs were prepared in the same manner as in Examples 1 to 7except that the polyurethane resin (c) was used and compounds shown inTable 2 were used as a lubricant, and evaluated in the same manner asabove. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                         Surface                                                                       roughness                                                                              Durability                                          No.        Lubricant   Ra (μm) 5° C.                                                                       45° C.                          ______________________________________                                        Example No.                                                                             8    Octyl stearate                                                                            0.018    3    8                                              9    Tridecyl    0.019    4    7                                                   stearate                                                                10    n-Butyl     0.017    3    5                                                   palmitate                                                               11    n-Butyl     0.018    3    6                                                   myristate                                                               12    n-Decyl     0.019    5    7                                                   myristate                                                               13    Myristyl    0.020    4    6                                                   palmitate                                                               14    n-Propyl    0.022    8    12                                                  laurate                                                                 15    Ethyl       0.024    10   15                                                  n-nonadecylate                                                 Comparative                                                                             4    Cetyl stearate                                                                            0.39     *    *                                    example No.                                                                             5    Stearyl stearate                                                                          0.24     *    *                                              6    Methyl      0.041    *    *                                                   pelargonate                                                              7    Liquid paraffin                                                                           0.062    *    *                                    ______________________________________                                    

EXAMPLES 16 TO 25 AND COMPARATIVE EXAMPLES 8 TO 15

Magnetic discs were prepared using polyurethane resins and lubricantsshown in Table 3 according to the methods of Examples 1 to 7, andevaluated in the same manner as above. The results are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                                       Surface                                             Polyurethane              roughness                                                                           Durability                           No.      resin  Lubricant          Ra (um)                                                                             5° C.                                                                     45° C.                     __________________________________________________________________________    Example No.                                                                          16                                                                              c      n-Butyl stearate/tridecyl stearate(1:1)                                                          0.018 5> 8                                        17       n-Butyl stearate/n-butyl palmitate(1:1)                                                          0.017 5> 7                                        18                                                                              b      Octyl stearate     0.019 5> 8                                        19       Tridecyl stearate  0.018 5> 7                                        20       n-Butyl palmitate  0.020 5> 7                                        21       n-Butyl myristate  0.018 5> 8                                        22                                                                              d      Octyl stearate     0.016 5> 9                                        23       Tridecyl stearate  0.017 5> 8                                        24       n-Butyl palmitate  0.017 5> 9                                        25       n-Butyl myristate  0.015 5> 7                                 Comparative                                                                           8                                                                              b      Cetyl stearate     0.29  *  *                                 example No.                                                                           9       Stearyl stearate   0.28  *  *                                        10       Methyl pelargonate 0.022 *  *                                        11       Liquid paraffin    0.070 *  *                                        12                                                                              d      Cetyl stearate     0.32  *  *                                        13       Stearyl stearate   0.26  *  *                                        14       Methyl pelargonate 0.018 *  *                                        15       Liquid paraffin    0.053 *  *                                 __________________________________________________________________________

EXAMPLES 26 TO 32 AND COMPARATIVE EXAMPLES 16 TO 19

    ______________________________________                                                                   Part by                                                                       weight                                             ______________________________________                                            Hexagonal barium ferrite (particle size, 0.08 μm)                                                       100                                              Polyurethane resin (described in Table 4)                                                                  25                                               Nitrocellulose (a product of Daiseru Co.)                                                                  5                                                Coronate L (a product of Nippon Polyurethane Co.)                                                          5                                                Al.sub.2 O.sub.3 (particle size, 0.1 μm)                                                                6                                                Carbon black (Ketzen black ®, a product of                                                             4                                                Lion Akzo Co., Ltd.)                                                          n-Butylcellosolve palmitate  3                                                Methyl ethyl ketone          120                                              Toluene                      120                                              Cyclohexane                  60                                           ______________________________________                                    

These materials were placed in a ball mill and milled together for 60hours to prepare a uniform dispersion. This dispersion was uniformlycoated onto a polyethylene terephthalate film of 75 μm in thickness sothat the dry thickness was 1.5 μm, dried and hardened by allowing thefilm to stand at a temperature of 60° C. for 24 hours. Magnetic discswere then stamped out of the film and evaluated as a magnetic recordingmedium. The results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________             Polyurethane resin                                                               Content of metal                                                                       Surface                                                                             Dynamic friction                                                                              Void                                           sulfonate group                                                                        roughness                                                                           coefficient                                                                            Durability                                                                           content                            No.      No.                                                                              (equivalent/10.sup.6 g)                                                                (μm)                                                                             5° C.                                                                      45° C.                                                                      5° C.                                                                     45° C.                                                                     (vol. %)                           __________________________________________________________________________    Example No.                                                                          26                                                                              A   20      0.021 0.21                                                                              0.23 20 18  4                                         27                                                                              B   47      0.018 0.21                                                                              0.22 8  5   3                                         28                                                                              C   85      0.017 0.19                                                                              0.20 5  8   3                                         29                                                                              D  150      0.018 0.15                                                                              0.18 5  6   3                                         30                                                                              E  250      0.018 0.15                                                                              0.17 5  7   4                                         31                                                                              F  378      0.018 0.16                                                                              0.18 5  6   5                                         32                                                                              G  495      0.020 0.18                                                                              0.23 8  15  6                                  Comparative                                                                          16                                                                              U   0       0.098 0.28                                                                              0.32 *  *   15                                 example No.                                                                          17                                                                              V   18      0.042 0.25                                                                              0.29 25 *   10                                        18                                                                              W  530      0.037 0.28                                                                              0.29 31 *   9                                         19                                                                              X  800      0.087 0.31                                                                              0.42 *  *   25                                 __________________________________________________________________________

EXAMPLES 33 TO 44 AND COMPARATIVE EXAMPLES 20 TO 28

    __________________________________________________________________________                                    Part by                                                                       weight                                        __________________________________________________________________________      Hexagonal barium ferrite (particle size, 0.10 (μm)                                                       100                                             Polyurethane resin (D)        25                                              Nitrocellulose (a product of Daiseru Kagaku Kogyo Co.)                                                      4                                               Coronate L (a product of Nippon Polyurethane Co.)                                                           6                                               Al.sub.2 O.sub.3 (Mohs' hardness, 9; particle size, 0.1                                                     Described                                       α-Fe.sub.2 O.sub.3 (Mohs' hardness, 6; particle size, 0.3                                             in Table                                        Brass powders (Mohs' hardness, 4; particle size, 0.2 μm)                                                 5                                               Carbon black (Ketzen Black ®; a product of Lion Akzo                                                    3                                               Co., Ltd.)                                                                    MEK                           125                                             Toluene                       125                                             Cyclohexane                   62                                              n-Butylcellosolve stearate    1.0                                             n-Butylcellosolve palmitate   2.0                                           __________________________________________________________________________

These materials were treated in the same manner as in Examples 26 to 32(Comparative examples 16 to 19) to prepare magnetic recording mediawhich were then evaluated in the same manner as above. The results areshown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                      Brass                                                                              Surface                                                                             Dynamic friction                                                                              Void                                      Al.sub.2 O.sub.3                                                                  α-Fe.sub.2 O.sub.2                                                           powders                                                                            roughness                                                                           coefficient                                                                            Durability                                                                           content                          No.      (%) (%)  (%)  (μm)                                                                             5° C.                                                                      45° C.                                                                      5° C.                                                                     45° C.                                                                     (Vol. %)                         __________________________________________________________________________    Example No.                                                                          33                                                                              2   0    0    0.022 0.22                                                                              0.24 15 18  3                                       34                                                                              5   0    0    0.018 0.15                                                                              0.18 5  8   3                                       35                                                                              10  0    0    0.018 0.16                                                                              0.21 5  8   4                                       36                                                                              20  0    0    0.021 0.17                                                                              0.20 18 22  6                                       37                                                                              0   2    0    0.026 0.25                                                                              0.28 20 24  3                                       38                                                                              0   5    0    0.022 0.20                                                                              0.21 6  8   4                                       39                                                                              0   10   0    0.023 0.22                                                                              0.24 6  10  4                                       40                                                                              0   20   0    0.028 0.20                                                                              0.25 20 24  8                                       41                                                                              1   2    0    0.019 0.19                                                                              0.20 5  7   4                                       42                                                                              3   1    1    0.018 0.18                                                                              0.18 6  8   4                                       43                                                                              3   4    1    0.020 0.19                                                                              0.22 5  8   5                                       44                                                                              5   0    1    0.018 0.18                                                                              0.18 5  6   4                                Comparative                                                                          20                                                                              0   0    3    0.019 0.28                                                                              0.29 *  *   4                                example No.                                                                          21                                                                              0   0    5    0.018 0.31                                                                              0.35 *  *   4                                       22                                                                              0   0    10   0.023 0.34                                                                              0.41 *  *   6                                       23                                                                              1   0    0    0.018 0.23                                                                              0.29 25 *   3                                       24                                                                              25  0    0    0.028 0.16                                                                              0.18 35 38  12                                      25                                                                              0   1    0    0.021 0.27                                                                              0.28 *  *   4                                       26                                                                              0   22   0    0.024 0.19                                                                              0.22 28 30  13                                      27                                                                              1   0    3    0.019 0.24                                                                              0.30 27 *   4                                       28                                                                              22  0    5    0.027 0.32                                                                              0.40 *  *   12                               __________________________________________________________________________     *The amounts of Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3 and brass powders are      expressed by wt. % based on hexagonal barium ferrite (this is also the        same with the following tables).                                         

EXAMPLES 45 TO 48 AND COMPARATIVE EXAMPLES 29 TO 34

Magnetic recording media were prepared in the same manner as in Examples33 to 44 except that the polyurethane resin (A) in Table 4 was used, andevaluated in the same manner as above. The results are shown in Table 6.

EXAMPLES 49 TO 52 AND COMPARATIVE EXAMPLES 35 TO 40

Magnetic recording media were prepared in the same manner as in Examples33 to 44 except that the polyurethane resin (G) in Table 4 was used, andevaluated in the same manner as above. The results are shown in Table 7.

COMPARATIVE EXAMPLES 41 TO 46

Magnetic recording media were prepared in the same manner as in Examples33 to 44 except that the polyurethane resin (V) in Table 4 was used, andevaluated in the same manner as above. The results are shown in Table 8.

COMPARATIVE EXAMPLES 47 TO 52

Magnetic recording media were prepared in the same manner as in Examples33 to 44 except that the polyurethane resin (W) in Table 4 was used, andevaluated in the same manner as above. The results are shown in Table 9.

                                      TABLE 6                                     __________________________________________________________________________    Polyurethane resin (A)                                                                          Brass                                                                              Surface                                                                             Dynamic friction                                                                              Void                                      Al.sub.2 O.sub.3                                                                  α-Fe.sub.2 O.sub.3                                                           powders                                                                            roughness                                                                           coefficient                                                                            Durability                                                                           content                          No.      (%) (%)  (%)  (μm)                                                                             5° C.                                                                      45° C.                                                                      5° C.                                                                     45° C.                                                                     (Vol. %)                         __________________________________________________________________________    Example No.                                                                          45                                                                              0   0    0    0.019 0.24                                                                              0.25 17 20  6                                       46                                                                              10  0    0    0.021 0.18                                                                              0.20  8 10  8                                       47                                                                              0   3    0    0.020 0.25                                                                              0.26 18 18  5                                       48                                                                              0   10   0    0.022 0.19                                                                              0.20 10  9  6                                Comparative                                                                          29                                                                              0   0    3    0.021 0.35                                                                              0.36 *  *   9                                example No.                                                                          30                                                                              0   0    10   0.024 0.32                                                                              0.34 *  *   12                                      31                                                                              1   0    0    0.018 0.28                                                                              0.30 32 *   6                                       32                                                                              0   1    0    0.020 0.27                                                                              0.31 35 *   5                                       33                                                                              25  0    0    0.027 0.19                                                                              0.20 40 42  12                                      34                                                                              0   25   0    0.029 0.20                                                                              0.22 35 30  13                               __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Polyurethane resin (G)                                                                          Brass                                                                              Surface                                                                             Dynamic friction                                                                              Void                                      Al.sub.2 O.sub.3                                                                  α-Fe.sub.2 O.sub.3                                                           powders                                                                            roughness                                                                           coefficient                                                                            Durability                                                                           content                                   (%) (%)  (%)  (μm)                                                                             5° C.                                                                      45° C.                                                                      5° C.                                                                     45° C.                                                                     (Vol. %)                         __________________________________________________________________________    Example No.                                                                          49                                                                              3   0    0    0.017 0.20                                                                              0.21 12 13  5                                       50                                                                              10  0    0    0.019 0.19                                                                              0.23  5  8  7                                       51                                                                              0   3    0    0.018 0.22                                                                              0.22 16 18  6                                       52                                                                              0   10   0    0.019 0.18                                                                              0.20  9 11  8                                Comparative                                                                          35                                                                              0   0    3    0.020 0.27                                                                              0.30 *  *   7                                example No.                                                                          36                                                                              0   0    10   0.024 0.29                                                                              0.33 *  *   8                                       37                                                                              1   0    0    0.016 0.24                                                                              0.26 *  *   5                                       38                                                                              0   1    0    0.016 0.25                                                                              0.29 *  *   5                                       39                                                                              25  0    0    0.022 0.19                                                                              0.21 32 40  12                                      40                                                                              0   25   0    0.023 0.20                                                                              0.23 25 30  13                               __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Polyurethane resin (V)                                                                          Brass                                                                              Surface                                                                             Dynamic friction                                                                              Void                                      Al.sub.2 O.sub.3                                                                  α-Fe.sub.2 O.sub.3                                                           powders                                                                            roughness                                                                           coefficient                                                                            Durability                                                                           content                          No.      (%) (%)  (%)  (μm)                                                                             5° C.                                                                      45° C.                                                                      5° C.                                                                     45° C.                                                                     (Vol. %)                         __________________________________________________________________________    Comparative                                                                          41                                                                              3   0    0    0.026 0.26                                                                              0.30 28 *    9                               example No.                                                                          42                                                                              10  0    0    0.029 0.25                                                                              0.30 32 *   13                                      43                                                                              0   3    0    0.028 0.28                                                                              0.32 30 *   10                                      44                                                                              0   10   0    0.032 0.26                                                                              0.28 29 *   15                                      45                                                                              0   0    3    0.032 0.32                                                                              0.35 *  *   12                                      46                                                                              0   0    10   0.038 0.38                                                                              0.36 *  *   18                               __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________    Polyurethane resin (W)                                                                          Brass                                                                              Surface                                                                             Dynamic friction                                                                              Void                                      Al.sub.2 O.sub.3                                                                  α-Fe.sub.2 O.sub.3                                                           powders                                                                            roughness                                                                           coefficient                                                                            Durability                                                                           content                          No.      (%) (%)  (%)  (μm)                                                                             5° C.                                                                      45° C.                                                                      5° C.                                                                     45° C.                                                                     (Vol. %)                         __________________________________________________________________________    Comparative                                                                          47                                                                              3   0    0    0.026 0.30                                                                              0.34 35 *    8                               example No.                                                                          48                                                                              10  0    0    0.032 0.28                                                                              0.31 32 *   12                                      49                                                                              0   3    0    0.029 0.32                                                                              0.33 *  *    9                                      50                                                                              0   10   0    0.035 0.30                                                                              0.31 30 *   15                                      51                                                                              0   0    3    0.036 0.34                                                                              0.38 *  *   15                                      52                                                                              0   0    10   0.040 0.34                                                                              0.35 *  *   21                               __________________________________________________________________________

What is claimed is:
 1. A magnetic recording medium which comprisesapplying a magnetic layer containing (A) magnetic fine particles ofplate-like hexagonal ferrite having a particle size of 0.3 μm or lessand an easy axis for magnetization in the direction vertical to the flatplate surface, (B) a polyurethane resin having a metal sulfonate groupof 20 to 500 equivalents/10⁶ g of the polymer and (C) one or moremembers selected from the esters of C₁₀ -C₁₈ fatty acids with alcoholshaving not more than 15 carbon atoms, the ratio of (C) to (A) being 0.1to 20 wt. %, to a non-magnetic substrate.
 2. A magnetic recording mediumas claimed in claim 1, wherein the magnetic layer contains (C) and (A)in a (C):(A) weight ratio of from 1.0 to 7.0 wt. %.
 3. A magneticrecording medium as claimed in claim 1, wherein the magnetic layercontains non-magnetic fine particles having a Mohs' hardness of 6 ormore and a particle size of 1.0 μm or less in a weight ratio to (A) of 2to 20 wt. %.
 4. A magnetic recording medium as claimed in claim 1,wherein said magnetic recording medium is a flexible and disc-like one.